Innate-like splenic marginal zone (MZ) and peritoneal cavity-derived (PEC) B1 B lineage cells share critical importance at the earliest stages of humoral immune defense by rapidly sensing pathogens and mounting primary antibody responses. However, the mechanistic basis for their robust responsivity to innate thymus- independent stimuli and involvement in disease pathogenesis is poorly understood. A multigene family termed FCR-like (FCRL1-6) encodes Ig-like transmembrane proteins with complex tyrosine-based immunoregulatory function that are preferentially expressed by B cells. Their immunologic significance is underscored by their evolutionary conservation and growing associations with human lymphoproliferative, autoimmune, infectious, and immunodeficiency disorders. Although these relationships implicate the FCRLs as pathogenic factors, studies thus far indicate that they generally inhibit B cell antigen-receptor (BCR) signaling. Several representatives have recently been found to interact with MHC-related proteins as well as immunoglobulins; however, the endogenous ligands and in vivo functions of most FCRL molecules expressed by B cells have not yet been defined. The long-term objective of our studies is to determine the role of the FCRL protein family in normal and perturbed immunobiology. FCRL5, the focus of this proposal, is uniquely expressed by innate-like MZ and B1 B cells in mice, has both ITAM-like and ITIM cytoplasmic sequences, and exerts dual-regulatory influence on BCR signaling by recruiting Lyn and SHP-1. Despite its discrete distribution, little is known about its impact or that of other FCRLs in innate immunity. The central hypothesis is that FCRL5 catalyzes innate T cell-independent responses in B cells through interactions with a recently defined counterpart. Based on supporting preliminary data three Specific Aims are proposed. The first Aim will determine the impact of FCRL5 deficiency on innate responses. The second Aim will dissect the mechanistic influence of FCRL5 in regulating innate-like B cell functions. In Aim 3 we will characterize structure/function interactions between FCRL5 and a newfound ligand. The proposed work is innovative because it harnesses conserved interspecies features and in vivo approaches that are expected to substantially shift our understanding of these receptors in innate immunity. The contributions of this study will be significant because they will provide new knowledge by integrating complex signaling and ligand relationships for FCRL5 in innate-like B cells and advancing our understanding of the physiologic impact of these interactions in regulating innate defense responses. These outcomes are anticipated to have a positive impact because they will establish a fundamental translational platform that provides insight into the pathogenic influence of FCRL molecules in immune disorders and reveals new avenues for disease prevention and treatment.